Radiosurgery and radiotherapy systems are radiation treatment systems that use external radiation beams to treat pathological anatomies (e.g., tumors, lesions, vascular malformations, nerve disorders, etc.) by delivering a prescribed dose of radiation (e.g., x-rays or gamma rays) to the pathological anatomy while minimizing radiation exposure to surrounding tissue and critical anatomical structures (e.g., the spinal chord). Both radiosurgery and radiotherapy are designed to necrotize the pathological anatomy while sparing healthy tissue and the critical structures.
Radiotherapy and radiosurgery systems include fixed-source gamma-ray treatment systems, and gantry-based and robotic-based x-ray treatment systems. In a fixed-source gamma-ray treatment system, multiple gamma ray beams from a distributed radioactive source (e.g., cobalt-60) are directed through bores in a hemispherical shield to a fixed point of convergence (isocenter). The patient is then moved relative to the isocenter to treat the pathology. In gantry-based x-ray systems, an x-ray beam source is attached to a gantry that moves around a center of rotation (isocenter) in a single plane. Each time an x-ray beam is delivered during treatment, the axis of the beam passes through the isocenter. As above, the patient is moved relative to the isocenter in order to treat the pathology. In robotic-based x-ray systems, the x-ray source has multiple degrees of freedom and the radiation treatment is not constrained to an isocenter.
In the x-ray treatment systems described above, the x-ray source is typically an electron linear accelerator (LINAC). A LINAC produces x-rays by bombarding a target with a high-energy electron beam. The x-rays are high-energy photons produced when the electron beam is intercepted by a target material having a high atomic number, such as tungsten for example. Two types of x-rays are produced. The first type is bremsstrahling (braking) radiation, given up by the electrons when they are slowed in the target. The second type is k-shell radiation, produced when atomic electrons transition between energy states after being excited by the bombarding electrons in the electron beam. Bremsstrahling radiation produces a continuous energy spectrum, while k-shell radiation has an energy signature that is a function of the target material. X-rays are produced continuously through the cross-section of a target as a nonlinear function of electron beam current and energy.
In a LINAC, electrons from an electron gun at a negative potential are accelerated to the target, which is held at a positive potential relative to the electron gun (e.g., at a system ground potential). The electrons are accelerated through a waveguide accelerator structure by applying a sinusoidal radio frequency (RF) voltage to a series of evacuated, tuned cavities that form the structure. If the dimensions of the cavities and the frequency of the RF voltage have the proper relationship, the electrons experience an accelerating field, gaining velocity until they approach the speed of light, and then increasing their relativistic mass as they travel through the structure.
Radio frequency LINACs used for radiotherapy or radiosurgery applications typically employ either a traveling wave accelerator or a standing wave accelerator. In a traveling wave accelerator, the RF electric field launches from one end of the structure and travels to the other end, where the unused portion of the power is absorbed externally or internally to the structure. Electrons are accelerated by riding the crests of the RF electric field as it travels through the waveguide structure. In a standing wave accelerator, the RF electric field is allowed to reflect from the ends of the accelerator structure, setting up a standing wave pattern similar to that of a vibrating string. The pattern has stationary minima (nodes) centered in every other cavity and sinusoidally time-varying fields in the intervening cavities that oscillate between positive and negative maxima. Electrons are accelerated in a standing wave structure by receiving a “push” from the electric field in a cavity with a time-varying field, coasting through a cavity with a near-zero field node, and then arriving at the next cavity at the optimum time to receive the next “push.” One variation of the standing wave accelerator includes the side-coupled standing wave accelerator. In a side-coupled standing wave accelerator, the nodal cavities (cavities with a near-zero RF field) are moved off axis and side-coupled with the accelerating cavities. This configuration allows the electrons to be continuously accelerated while shortening the overall length of the accelerator.
Conventional LINACS, such as those used in gantry-type radiosurgery systems, are S-Band designs (S-Band designates frequencies between 2 gigahertz and 4 gigahertz), typically operating at an RF frequency of approximately 3 gigahertz (3 GHz) and accelerating electrons to energies ranging from 4 to 20 million electron volts (MeV), with a typical value being approximately 6 Mev. At 3 Ghz, the RF cavities of the accelerator are relatively large, and the manufacturing tolerances of the RF cavities are relatively easy to meet. However, the accelerator, which is normally fabricated from copper for good electrical and thermal conductivity, is large and heavy. As a result, the LINAC requires rigid support within the gantry. FIG. 1 illustrates a cross-sectional schematic of a conventional gantry system 100. The LINAC 101 is mounted parallel to the axis of rotation of the gantry. A high power klystron oscillator 102, mounted in a stationary part of the system (stand 103), provides the 3 Ghz RF energy to the LINAC 101 through pressurized waveguides 104, a circulator 105 (to absorb energy reflected from the LINAC) and a rotating waveguide joint 106. The waveguide is pressurized by pressure system 107 with an insulating gas (e.g., sulfur hexafluoride) to prevent electrical breakdown in the waveguide from high RF electric fields. A water cooling system 108 provides temperature controlled water to the klystron and LINAC to maintain frequency stability. Temperature induced dimensional changes in the LINAC can detune the RF cavities and impair the acceleration process. Some systems also include an automatic frequency control (AFC) circuit 109 to fine tune the frequency of the klystron to match the optimum frequency of the LINAC. System 100 also includes a pulsed power supply 110 to synchronously pulse the klystron and the electron gun 111 in the LINAC. A remote control console 112 coordinates the overall functioning of the system.
Controlling the x-ray dose-rate and total radiation dose is important in medical LINAC applications, particularly in radiotherapy and radiosurgery where excessive radiation exposure can damage healthy tissue.
One conventional approach to controlling electron beam energy, used in the gantry system of FIG. 1, is the use of a magnetic field to bend and focus the electrons in a treatment head 115. FIG. 2 is a cross-sectional view of the treatment head 115. The treatment head includes a bending magnet assembly 116 that is coupled to the LINAC 101 and maintained at a vacuum with the LINAC by a vacuum system 113. Magnets in the bending magnet assembly (not shown) generate a magnetic field transverse to the path of the electron beam 117 as it exits the LINAC. The force exerted on an electron in a magnetic field is proportional to the charge of the electron, the vector velocity of the electron, and the vector magnetic field, and is directed at a 90 degree angle to both the magnetic field and the instantaneous trajectory of the electron. As a result, the electrons in the beam are bent in a 270 degree orbit and are directed downward when they strike the x-ray target 118. The strength and shape of the magnetic field is designed such that only those electrons in a specified energy range (i.e., velocity range) will strike the target and generate x-rays. Electrons with energies above or below the specified range take different trajectories and are intercepted by absorbing materials located outside of the target trajectory. Electrons exit the vacuum of the accelerator and bending assembly through a window 120 and strike the x-ray target 118. The x-ray beam then passes through an ionization chamber 121, where the ionization current produced by the x-rays provides an indirect measure of the radiation dose applied to the patient.
One problem in conventional systems is that the large number of electrons which exit the vacuum of the accelerator structure can interact with the ambient atmosphere of the operating room to generate ozone in collisions with oxygen molecules. Ozone is a powerful oxidizer, is corrosive to many common operating room materials and can cause lung irritation and damage in humans.